Planar display devices are widely applied due to numerous advantages such as thin bodies, energy saving, radiation-free, etc. The current planar display devices mainly comprise liquid crystal display (LCD) devices and organic light-emitting diode (OLED) display devices.
OLED display devices have excellent properties including self-luminous, no backlight, high contrast, thin thickness, wide viewing angle, fast response, applicable for flexible panels, wide temperature range for use, simpler structure and process, etc. Accordingly, the OLED display devices are considered as the emerging application of the next generation of flat panel display technologies. An OLED display device generally includes a substrate, an anode disposed on the substrate, an organic light-emitting layer disposed on the anode, and a cathode disposed on the organic light-emitting layer. When it operates, the holes from the anode and the electrons from the cathode are emitted to the organic light-emitting layer. The electrons and holes are combined to generate excited electron-hole pairs. The excited electron-hole pairs are converted from the excited state to the ground state so as to achieve light-emitting.
According to driving modes, the OLED display device can be classified into two main types: a passive matrix OLED (PMOLED) type and an active matrix OLED (AMOLED) type. They are two categories of direct addressing and thin film transistor (TFT) matrix addressing. Among them, the AMOLED has pixels arranged in an array. It belongs to the active display type with high light-emitting efficiency and excellent performance.
Thin film transistors commonly used in the prior art include amorphous silicon (a-Si) thin film transistors, low temperature poly-silicon (LTPS) thin film transistors, and oxide semiconductor thin film transistors. The oxide semiconductor thin film transistors have higher electron mobility. Moreover, compared with the LTPS thin film transistors, the fabrication process of the oxide semiconductor thin film transistors is simpler and has higher compatibility with amorphous silicon thin film transistor processes. Therefore, the oxide semiconductor thin film transistors have been widely used.
In the oxide semiconductor thin film transistor, the oxygen content in its active layer (i.e., the oxide semiconductor layer) has a great influence on the device characteristics. Currently, the active layer in the oxide semiconductor thin film transistor is generally designed as a single film. The active layer with this kind of single film is formed through one deposition process. If the oxygen flow rate during the deposition process is larger, the oxygen content of the active layer will be too high. As a result, the conductivity becomes poor, and the mobility of the device is reduced. On the other hand, if the oxygen flow rate during the deposition process is smaller, the oxygen content in the active layer is too low, and there are more oxygen vacancies. As a result, the interface defects at the contact interface between the active layer and the buffer layer and the contact interface between the active layer and the gate insulating layer increase. The device stability becomes worse.